White Matter Microstructural Changes Using Ultra-Strong Diffusion Gradient MRI in Adult-Onset Idiopathic Focal Cervical Dystonia

Background and Objectives Adult-onset idiopathic focal cervical dystonia (AOIFCD) involves abnormal posturing of the cervical musculature and, in some individuals, an associated head tremor. Existing neuroimaging studies have implicated key motor networks. However, measures used to date lack specificity toward underlying pathophysiologic differences. We aim to assess white matter motor pathways for localized, microstructural differences, which may aid in understanding underlying mechanisms. Methods Individuals diagnosed with AOIFCD and an age- and sex-matched control group were prospectively recruited through the Welsh Movement Disorders Research Network. All participants underwent in-depth clinical phenotyping and MRI (structural and diffusion sequences) using ultra-strong diffusion gradients. Tractography (whole-tract median values) and tractometry (along tract profiling) were performed for key white matter motor pathways assessing diffusion kurtosis imaging (DKI), neurite orientation dispersion and density imaging (NODDI), and standard model parameters. Groups were compared using linear model analysis with Bonferroni multiple comparison correction. Results Fifty participants with AOIFCD and 30 healthy control participants were recruited, with 46 with AOIFCD and 30 healthy controls included for analysis (33 without head tremor, 13 with head tremor). Significant differences were observed in the anterior thalamic radiations (lower mid-tract fractional anisotropy [estimate = −0.046, p = 3.07 × 10⁻³], radial kurtosis [estimate = −0.165, p = 1.42 × 10⁻⁴], f–intra-axonal signal fraction [estimate = −0.044, p = 2.78 × 10⁻³], p₂ orientation coherence [estimate = −0.043, p = 1.64 × 10⁻³], higher Orientation Dispersion Index [ODI, estimate = 0.023, p = 2.22 × 10⁻³]) and thalamopremotor tracts (higher mid-tract mean kurtosis [estimate = 0.064, p = 7.56 × 10⁻⁴], lower Neurite Density Index [estimate = 0.062, p = 2.1 × 10⁻³], higher distal tract ODI [estimate = 0.062, p = 3.1 × 10⁻³], lower f [estimate = −0.1, p = 2.3 × 10⁻³], and striatopremotor tracts [proximal lower f: estimate = −0.075, p = 1.06 × 10⁻³]). These measures correlated with clinical measures: dystonia duration (right thalamopremotor distal ODI: r = −0.9, p = 1.29 × 10⁻¹⁴), psychiatric symptoms (obsessive compulsive symptoms: left anterior thalamic radiation p₂ r = 0.92, p = 2.797 × 10⁻¹¹), sleep quality (Sleep Disorders Questionnaire Score: left anterior thalamic radiation ODI: r = −0.84, p = 4.84 × 10⁻¹¹), pain (left anterior thalamic radiation ODI: r = −0.89, p = 1.4 × 10⁻¹³), and cognitive functioning (paired associated learning task p2, r = 0.94, p = 6.68 × 10⁻²⁰).